Enhanced digital diagnostics for optical PAM apparatus

ABSTRACT

A Pulse Amplitude Modulated (PAM) optical device utilizing multiple wavelengths, features a communications interface having enhanced diagnostics capability. New registers are created to house additional diagnostic information, such as error rates. The diagnostic information may be stored in raw form, or as processed on-chip utilizing local resources.

CROSS-REFERENCE TO RELATED APPLICATION

The instant nonprovisional patent application is a Continuation of U.S.patent application Ser. No. 15/490,555 filed Apr. 18, 2017, which is acontinuation of U.S. patent application Ser. No. 15/069,702 filed onMar. 14, 2016 (now U.S. Pat. No. 9,660,730 issued May 23, 2017), whichis a Continuation-in-Part of parent U.S. patent application Ser. No.14/881,401 filed Oct. 13, 2015 and incorporated by reference in itsentirety herein for all purposes.

BACKGROUND

The present invention relates to telecommunication techniques. Moreparticularly, the present invention provides enhanced diagnosticcapabilities for a Pulse Amplitude Modulated (PAM) optical device,although other applications are possible.

Over the last few decades, the use of communication networks exploded.In the early days Internet, popular applications were limited to emails,bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Progress in computer technology (and the continuation of Moore's Law) isbecoming increasingly dependent on faster data transfer between andwithin microchips. Optical interconnects may provide a way forward, andsilicon photonics may prove particularly useful, once integrated on thestandard silicon chips. 40-Gbit/s and then 100-Gbit/s data rates DWDMoptical transmission over existing single-mode fiber is a target for thenext generation of fiber-optic communication networks. Everything isokay up to 10 Gbits/s, but beyond that, distortion and dispersion taketheir toll.

In order to meet the challenges of faster data communications withaccuracy maintained, the use of multiple channels has been explored.This, however, leads to a more complex optical communications device,with a corresponding greater variety and volume of available diagnosticinformation. Therefore, improved devices with enhanced features aredesired.

SUMMARY

A Pulse Amplitude Modulated (PAM) optical device utilizing multiplewavelengths, features a communications interface having enhanceddiagnostics capability. New registers are created to house additionaldiagnostic information, such as error rates and other data. Thediagnostic information may be stored in raw form, or as processedon-chip utilizing local available resources.

In general, the present invention provides a PAM optical deviceincluding enhanced diagnostics capability. The present inventionachieves these benefits and others in the context of known opticaltechnology. However, a further understanding of the nature andadvantages of the present invention may be realized by reference to thelatter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a simplified diagram illustrating interaction between a hostand an optical communication module according to an embodiment.

FIG. 2 is a simplified diagram illustrating an optical communicationmodule according to an embodiment.

FIG. 3 is a simplified diagram illustrating further details of anoptical communication module.

FIG. 4 shows a parameter level histogram.

FIG. 5 shows post-Analog to Digital Conversion (ADC) sample capture.

FIG. 6 shows post-Feed Forward Equalization (FFE) sample capture, withslicer levels.

FIG. 7 is a sample plot of FFE Tap coefficient versus tap position.

FIG. 8 is a plot of reflection-canceller coefficients over tap position.

FIG. 9 is an extracted overall link pulse response highlighting residualInter-Symbol Interference (ISI) components.

FIG. 10 is a pie chart breaking down Bit Error Rate (BER)/SNR to Noise,Non-linearities (NL), Residual ISI and low-frequency DC baseline errors.

FIG. 11 shows a host eye-scan feature.

FIG. 12 is a table showing a register overview for a new page storingadditional diagnostic parameters for monitoring.

FIG. 13 is a table showing registers in the table of FIG. 12 foralarm/warning latches.

FIG. 14 is a table showing real-time value registers in the table ofFIG. 12.

FIG. 15 is a table showing interrupt mask registers in the table of FIG.12.

FIG. 16 is a table showing parameter configuration registers in thetable of FIG. 12.

FIG. 17 is a table showing details of the parameter configuration ofFIG. 16.

FIG. 18 is a table showing parameter type enumeration.

FIG. 19 is a simplified diagram illustrating mapping by the opticalmodule.

FIG. 20A is a table showing lane mapping registers in the table of FIG.12.

FIG. 20B is a table showing lane mapping enumeration.

FIG. 21 is a table showing other configuration registers in the table ofFIG. 12.

FIG. 22 is a table showing threshold registers in the table of FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION

In the last decades, with advent of cloud computing and data center, theneeds for network servers have evolved. For example, the multiple-levelswitch/router configuration that have been used for a long time is nolonger adequate or suitable, as distributed applications require flatternetwork architectures, where server virtualization that allows serversto operate in parallel. For example, multiple servers can be usedtogether to perform a requested task. For multiple servers to work inparallel, it is often imperative for them to be share large amount ofinformation among themselves quickly, as opposed to having data goingback forth through multiple layers of network architecture (e.g.,network switches, etc.).

Leaf-spine type of network architecture is provided to better allowservers to work in parallel and move data quickly among servers,offering high bandwidth and low latencies. Typically, a leaf-spinenetwork architecture uses a top-of-rack switch that can directly accessinto server nodes and links back to a set of non-blocking spine switchesthat have enough bandwidth to allow for clusters of servers to be linkedto one another and share large amount of data.

In a typical leaf-spine network today, gigabits of data are shared amongservers. In certain network architectures, network servers on the samelevel have certain peer links for data sharing. Unfortunately, thebandwidth for this type of set up is often inadequate. It is to beappreciated that embodiments of the present invention utilizes PAM(e.g., PAM4, PAM8, PAM12, PAM16, etc.) in leaf-spine architecture thatallows large amount (up terabytes of data at the spine level) of data tobe transferred via optical network.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

A communications module that is capable of accurate transmission of datain electronic and digital form at high speeds, is complex. It offersmany types of data useful for diagnostic purposes.

Moreover, the optical communications module may be designed to interactwith a host according to particular industry standard(s). For example,incorporated by reference herein for all purposes, are the followingexisting standard MSA documents:

-   -   SFF-8636, Specification for Common Management Interface, Rev 2.4        (Nov. 7, 2014);    -   SFF-8024, Specification for Small Form Factor (SFF) Committee        Cross Reference to Industry Products, Rev 2.8 (Feb. 9, 2015)

The information present in these documents, however, may not providesufficient diagnostics data to identify and/or manage a complex opticalPAM apparatus. Accordingly, embodiments provide enhanced digitaldiagnostics capability to an optical PAM apparatus, for example throughthe use of new register definitions and values added thereto.

FIG. 1 shows a simplified view of a communications interface 100offering enhanced diagnostic capabilities according to an embodiment. Inparticular, communications interface 100 is in communication with host102.

The communications interface includes an integrated circuit 104comprising a non-transitory computer readable storage medium 105including existing register(s) 106 that are configured to store datarelating to its operation. The structure/organization of those existingregisters, and the information stored therein, may be dictated accordingto an industry standard, for example the SFF-8636 and/or SFF-8024standards mentioned above.

Those existing registers designed according to the industry standards,however, may be insufficient to store the volume of detailed informationpotentially useful for diagnostics of the advanced communicationsinterface.

Accordingly, embodiments introduce to memory 105, new registers 108 thatare configured to house additional information 110 relevant fordiagnostic purposes. Multiple examples of such additional informationare provided below, including but not limited to:

-   -   signal-to-noise ratios;    -   bit error rates;    -   frame errors;    -   level histogram parameters;    -   channel maps;    -   digital signal processing (DSP) parameters;    -   per-lane DSP Debug Memory outputs;    -   thermoelectric cooler (TEC) currents;    -   eye diagrams;    -   grid channel number;    -   wavelength error;    -   thresholds/Interrupts/Masks;    -   transceiver IC control; and    -   others.

The corresponding host should recognize, allow access to, and desirablyfully support these added values, in order to reap the full diagnosticbenefit of this enhanced capability.

Under some circumstances, the new registers may store the additionaldata in raw form (e.g., an error instance) as directly received fromvarious components. That raw data may be ultimately transmitted to thehost for further processing in order to produce finished datacorresponding to recognized diagnostic parameters (e.g., an error rateover a time increment).

Under other circumstances, the new registers may store data in afinished form that is already recognizable as being relevant fordiagnostic purposes. Such processing may take place in the host, withthe processed data being returned to the interface.

Alternatively, in certain embodiments the raw data 132 from an opticalmodule element 130 may be processed directly within the module itself,utilizing existing, locally available processing resources. This isgenerally referenced in FIG. 1 as engine 120.

FIG. 2 shows a top level diagram illustrating elements of the opticalcommunications module 200. In particular, communication between themodule and the host 202 is run via a master logic system (MLS) 204,which can be a microcontroller, Field-Programmable Gate Array (FPGA), orsimilar system.

The optical communications module further comprises a PAM driver 206, aswell as a PAM transceiver (Xcvr) chip 208. The MLS receives data fromthese components via the analog and digital diagnostics communicationsinterfaces (collectively referenced here as 210).

The MLS may then translate the raw data from those chips into “humanreadable” form for presentation to the host via communications interface220 (e.g., using the I2C standard or others). This processed diagnosticdata can comprise eye diagrams, histograms, and others etc.

Thus, the MLS gathers data from all the other blocks and reports it tothe host via the communications interface. While much of the diagnosticinformation comes from the PAM transceiver, data from other sources canbe harvested as well.

For example, diagnostic information can be collected at the systemlevel, or can come from other specific components of the communicationmodule—e.g., including but not limited to the PAM driver 206, laser/TEC212, photodetector 214, linear transimpedance amplifier (TIA) 216, andothers, for example as are described below in FIG. 3.

Data potentially valuable for enhanced diagnostics purposes, may becommunicated internally from the device to the host for analysis. And,as described later below, debugging data may offer a particularly usefulsource of such information.

In particular, FIG. 2 shows a debugging memory 222 that can be used toprovide a stream 205 of data for post-processing and diagnosticpurposes. This data collected at the debugging memory, can be passed viathe communications interfaces to diagnostic logic residing in the MLSwhich includes a new register 299.

In FIG. 2, the MLS 204 and PAM Xcvr 208 are connected via acommunications interface 210. The debug data is stored within the PAMXcvr, but must be transmitted to the host 202, which is not directlyconnected.

An embodiment of a method for moving the diagnostic data from the PAMXcvr to the host is as follows. First, the host writes a request fordata into a register at the MLS using the serial communicationsinterface 220.

The MLS then responds by querying the PAM Xcvr 208 for data related tothe request. The data is broken up into blocks (e.g., of 120 bytes)since the MLS has limited memory capacity.

The host indicates which block of data is to be read by writing to aregister. The host also indicates how much data is expected in thecurrent block.

Next, the MLS sets the value of a separate register to indicate that thedata is ready for download to the host.

The host then polls the data-ready register until it is informed thatthe data is ready.

The host then reads the data cache within the MLS via the serialcommunications interface.

The request/response structure could be as follows, kept within a128-byte “page” as is typical of the existing standards:

-   -   Byte 1: Feature Number (e.g., 0=Histogram data, 1=Eye Scan data,        etc.)    -   Byte 2: Block length (up to 120 bytes of data per block)    -   Bytes 3-4: Block Number (0-65535, each block is 120 bytes of        data, supporting up to 7864320 bytes per feature)    -   Bytes 5-124: Data    -   Bytes 125-128: Data error checking (e.g., CRC-32 or other error        checking parameter)

The feature number, and corresponding number and length of data blockscould be specified in advance.

It is noted that FIG. 2 represents a simplified diagram showing only asingle optical channel with PAM-4 encoding. However, this is notrequired and alternative embodiments may utilize different schemes.

FIG. 3 is a simplified diagram illustrating details of an embodiment ofan optical communication module 300. This diagram is merely one example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The communication module 300 includes transmitter element310 and a receiver element 320. The transmitter element 310 comprises areceiver 311, encoder 312, and PAM modulation driver 313.

In an embodiment, the communication module 300 is configured to receiveincoming data at through four channels, where each channel is configuredat 25 gigabits/s and configured as a PAM-2 format. Using the transmitterelement 310, modulator 316, and the laser 314, the communication module300 processes data received at 25 gigabits/s from each of the fourincoming channels, and transmits PAM modulated optical data stream at abandwidth of 100 gigabits/s. It is to be appreciated that otherbandwidths are possible as well, such as 40 Gbps, 400 Gbps, and/orothers.

As shown the transmitter element 310 receives 4 channels of data. It isto be appreciated that other variants of pulse-amplitude modulation(e.g., PAM4, PAM8, PAM12, PAM16, etc.), in addition to PAM-2 format, maybe used as well. The transmitter element 310 comprises functional block311, which includes a clock data recovery (CDR) circuit configured toreceive the incoming data from the four communication channels. Invarious embodiments, the functional block 311 further comprisesmultiplexer for combining 4 channels for data. For example, data fromthe 4 channels as shown are from the PCE-e interface 650. For example,the interface to host 350 is connected to one or more processors. In aspecific embodiment, two 2:1 multiplexers are employed in the functionalblock 311. For example, the data received from the four channels arehigh-speed data streams that are not accompanied by clock signals. Thereceiver 311 comprises, among other things, a clock signal that isassociated with a predetermined frequency reference value. In variousembodiments, the receiver 311 is configured to utilize a phase-lockedloop (PLL) to align the received data.

The transmitter element 310 further comprises an encoder 312. As shownin FIG. 3, the encoder 312 comprises a forward error correction (FEC)encoder. Among other things, the encoder 312 provides error detectionand/or correction as needed. For example, the data received may be aPAM-2 format as described above. The received data comprises redundancy(e.g., one or more redundant bits) helps the encoder 312 to detecterrors. In a specific embodiment, low-density parity check (LDPC) codesare used. The encoder 312 is configured to encode data received fromfour channels as shown to generate a data stream that can be transmittedthrough optical communication link at a bandwidth 100 gigabits/s (e.g.,combining 4 channels of 25 gigabits/s data). For example, each receivedis in the PAM-2 format, and the encoded data stream is a combination offour data channels and is in PAM-8 format. Data encoding and errorcorrection are used under PAM format.

The PAM modulation driver 313 is configured to drive data stream encodedby the encoder 312. In various embodiments, the receiver 311, encoder312, and the modulation driver 313 are integrated and part of thetransmitter element 310. Details regarding an example of a PAMmodulation driver according to particular embodiments, are disclosed inU.S. Nonprovisional patent application Ser. No. 14/798,322, filed Jul.13, 2015 and incorporated by reference in its entirety herein for allpurposes.

The PAM modulator 316 is configured to modulate signals from thetransmitter module 310, and convert the received electrical signal tooptical signal using the laser 314. For example, the modulator 316generates optical signals at a transmission rate of 100 gigabits persecond. It is to be appreciated that other rate are possible as well,such as 40 Gbps, 400 Gbps, or others. The optical signals aretransmitted in a PAM format (e.g., PAM-8 format, PAM12, PAM 16, etc.).In various embodiments, the laser 314 comprises a distributed feedback(DFB) laser. Depending on the application, other types of lasertechnology may be used as well, as such vertical cavity surface emittinglaser (VCSEL) and others.

This particular communication module 300 is configured for bothreceiving and transmitting signals. A receiver element 320 comprise aphoto detector 321 that converts incoming data signal in an opticalformat converts the optical signal to an electrical signal. In variousembodiments, the photo detector 321 comprises indium gallium arsenidematerial. For example, the photo detector 321 can be asemiconductor-based photodiode, such as p-n photodiodes, p-i-nphotodiodes, avalanche photodiodes, or others. The photo detector 321 iscoupled with an amplifier 322. In various embodiments, the amplifiercomprises a linear transimpedance amplifier (TIA). It is to beappreciated by using TIA, long-range multi-mode (LRM) at high bandwidth(e.g., 100 Gb/s or even larger) can be supposed. For example, the TIAhelps compensate for optical dispersion in electrical domain usingelectrical dispersion compensation (EDC). In certain embodiments, theamplifier 322 also includes a limiting amplifier. The amplifier 322 isused to produce a signal in the electrical domain from the incomingoptical signal. In certain embodiments, further signal processing suchas clock recovery from data (CDR) performed by a phase-locked loop mayalso be applied before the data is passed on.

The amplified data signal from the amplifier 322 is processed by theanalog to digital converter (ADC) 323. In a specific embodiment, the ADC323 can be a baud rate ADC. For example, the ADC is configured toconvert the amplified signal into a digital signal formatted into a 100gigabit per second signal in a PAM format. The functional block 324 isconfigured to process the 100 Gb/s data stream and encode it into fourat streams at 25 Gb/s each. For example, the incoming optical datastream received by the photo detector 321 is in PAM-8 format at abandwidth of 100 Gb/s, and at block 324 four data streams in PAM-2format is generated at a bandwidth of 25 Gb/s. The four data streams aretransmitted by the transmitter 325 over 4 communication channels at 25Gb/s.

It is to be appreciated that there can be many variations to theembodiments described in FIG. 3. For example, different number ofchannels (e.g., 4, 8, 16, etc.) and different bandwidth (e.g., 10 Gb/s,40 Gb/s, 100 Gb/s, 400 Gb/s, 3.2 Tb/s, etc.) can be used as well,depending on the application (e.g., server, leaf switch, spine switch,etc.).

In operation, the communication module 300 sends optical signal toanother communication interface. More specifically, the transmittermodule of one network interface sends signals over optical network tothe receiver module of another network interface. More specifically,electrical signals are modulated and converted to optical signals. Forexample, the PAM modulation driver 313 sends PAM modulated electricalsignals to the PAM modulator 316, which, together with the laser source314, sends modulated optical signals out. It is to be appreciated thatmodulated optical signals according to embodiments may be modulated bothin amplitude and phase.

EXAMPLES

A variety of features of an optical communication module may beavailable for capture and storage in order to provide enhanceddiagnostics purposes. One such feature is grid channel number.

This grid channel number feature indicates the base InternationalTelecommunications Union (ITU) grid channel number (center of the twoλs). A possible (but not required) location for the new register storingthis added information, is 20.

Another feature available for diagnostics is wavelength error. Thisfeature describes error of laser frequency. Possible (but not required)locations for the new registers for this additional information, are58-59 (λ1), 60-61 (λ2).

Still another possible feature available for diagnostic use, isthermoelectric cooler (TEC) current, e.g., in units of 0.1 mA. Thisrepresents generally is the amount of current flowing through the TEC,which is used to stabilize the frequency of the laser. Too much TECcurrent may be problematic, as the device will not be able to controlthe temperature/frequency of the laser. Possible (but not required)locations for the new registers storing this additional information, are62-63 (λ1), 64-65 (λ2).

Residual dispersion is another feature that may be made available forenhanced diagnostics according to embodiments. This feature may be basedon Finite Impulse Response (FIR) filter settings. Possible (but notrequired) locations for this new register are 66-67 (λ1), 68-69 (λ2).

The pre-Forward Error Correction (FEC) Bit Error Rate (BER) is anotherpossible diagnostic feature that is made available according toparticular embodiments. This feature represents a corrected error countfrom FEC.

A possible (but not required) location for this new register is 78-79(λ1), 80-81 (λ2). Note, it is also possible to provide per-level BER byadding more registers.

Upon encountering a volume of corrected errors beyond a certain limit,FEC techniques may be unable to continue functioning with accuracy.Thus, a diagnostic technique according to embodiments may determine thatthe FEC is above that limit, and then estimate a BER based uponSignal-to-Noise Ratio (SNR).

Frame errors represent yet another possible feature that can be madeavailable by enhanced diagnostics. Such frame error may be characterizedas module frame error count (e.g., a number of frames having at leastone error). A possible (but not required) location for this new registeris 30-31.

Under certain circumstances, however, such a raw error count may offerlimited value in diagnosing performance issues. Accordingly, embodimentsmay provide frame error other terms. One such alternative expression oferror may be in errored seconds. That is, the diagnostic function maycount a number of seconds in which at least one error is present. Thiserrored seconds measure can aid in diagnosing error sources arising overonly particular time periods.

Another possible available enhanced diagnostic feature comprisesthresholds/interrupts/masks. In particular, relatively simple (e.g.,monotonic—acceptable/unacceptable levels) diagnostic features that ableto be implemented directly on-chip, can permit the implementation ofthresholds and masks similar to other registers under the SFF-8636standard. For example, an on-chip function could track raw error countand determine therefrom a BER. A relatively simple calculation couldthen determine whether that BER exceeds a threshold, with error countsexceeding that threshold triggering some further action for diagnosticpurposes.

Still another possible feature available for advanced diagnostics istransceiver (Xcvr) IC control. This can comprise various bit-levelcontrol over the PAM IC, including but not limited to one or more of:

-   -   FEC enable/disable;    -   FEC mode.

It is further noted that diagnostic features provided according tovarious embodiments, may facilitate the starting and stopping ofcounters (e.g., counters for BER, frame errors SNR, etc.). Such lowlevel management of counter activity within the chip may promoteresponsive diagnostic behavior without imposing an excessive burden oncommunication resources (e.g., with the host to frequently start/stopcounters), and/or memory resources (e.g., to store counts above andbeyond those pertinent to diagnosis of a particular issue).

Yet another diagnostic feature can be a channel map. This feature showsmapping of electrical lanes to optical lane/bit position. Examples caninclude but are not limited to:

-   -   electrical lane 1→Optical Lane 1 Bit 1 MSB);    -   electrical lane 2→Optical Lane 1 Bit 0 LSB);    -   electrical lane 3→Optical Lane 2 Bit 1 MSB;    -   electrical lane 4→Optical Lane 2 Bit 0 LSB.

This channel mapping may be useful to accommodate embodiments that allowuser-configurable allocation of electrical lanes to optical lane/bitposition. Such a configurable device may be useful to promoteinteroperability with external devices (e.g., as potentially suppliedfrom other manufacturers).

Still other features may be available to offer enhanced diagnosticscapability for a PAM optical module. For example, SNR may be accessed.

A SNR monitor may be based upon based upon an output estimating MeanSquare Error (MSE), based on at least two error signals from the DigitalSignal Processing. The MSE read-out can be translated to SNR in dB,which is the most basic link health indicator. Possible (but notrequired) locations for such new registers can be 70-71 (λ1), 72-73(λ2).

Level histogram parameter is another possible diagnostic feature. Thisrepresents measurement of margin of level slicer or other parameters,determined via a level histogram.

For example, FIG. 4 shows a PAM4/NRZ Eye Histogram collected bypopulating post-EQ sample hits over N bins. Possible (but not required)new register numbers for this feature are 74-75 (λ1), 76-77 (λ2).

In this particular exemplary histogram of FIG. 4, a single bin saturatesat 2^20 hits, where N=160 in PAM4 mode. This affords sufficientresolution on the sample distribution. Examples of such sample hits caninclude but are not limited to Feed Forward Equalizer (FFE) output,FFE+Decision Feedback Equalizer (DFE) output, etc. based on DigitalSignal Processing (DSP) mode.

As shown in FIG. 4, such a histogram may be useful for implementingsimple diagnostic efforts such as identifying a margin to slider.However, the histogram may also be valuable for performing other, morecomplex diagnostic efforts. Examples can include but are not limited to,eye-symmetry analysis, and analysis of the tail of the histogram toextrapolate BER, etc.

Still another possible diagnostic feature which may be made available,is per-lane DSP debug memory. Such an on-chip per-lane debug memory maybe used for capturing data at various probe points within the DSP. Ofparticular note are the Analog to Digital Conversion (ADC) output andFFE output.

FIG. 5 shows a post-ADC sample capture. The ADC output can be used toexamine characteristics of incoming data. Usually the “eye” would beclosed, but this data is still useful for post-processing when combinedwith read-out of other DSP parameters.

FIG. 6 shows post-FFE sample capture, with slicer levels. Reading outthe captured FFE output, combined with the slicer information can revealinformation not only about the eye-opening and margins, but also theprofile of the error events if any. This can be a useful tool indebugging FEC performance.

Digital Signal Processing (DSP) parameters represent anotherfunctionality useful in enhanced diagnostics. A number of different DSPparameters may be available to read-out through registers newly-createdfor this purpose.

One example of a DSP parameter potentially useful for diagnosticefforts, is FFE Tap coefficient. FIG. 7 is a sample plot of suchcoefficients versus tap position.

Other potentially DSP parameters potentially useful for diagnosticpurposes, include but are not limited to PAM4 Levels, slicer thresholds,and DFE Tap coefficients.

Reflection-Canceller coefficients may also be useful for diagnosticpurposes. FIG. 8 provides a sample plot of such coefficients over tapposition.

Digital Timing Loop (DTL) parameters may also be useful. The Kfaccumulator value can be translated to extract lane frequency PPMoffset. The DTL code readout can be accumulated to extract DTLphase/tracking noise.

Analog Front-End Gain and/or Continuous Time Linear Equalizer (CTLE)Equalization code may also be provided for diagnostic purposes. Thisallows assessment of the peak-peak value of the incoming signal. DC gainand high-frequency analog boost may be applied to the incoming signal.

Still other functions that may be made available for diagnosticpurposes, include analog Front-End Gain, DC offset, and timing mismatcherrors.

The above DSP parameter read-outs, combined with debug memory datacaptures can be used for deeper link budget analysis. For example, anoverall link pulse response can be extracted which highlights any“linear” residual Inter-Symbol Interference (ISI) components. This isshown in FIG. 9.

Through further post-processing of the data, a finite link BER/SNR canbe eventually broken-down to: Noise, Non-linearities (NL), Residual ISIand low-frequency DC baseline errors, etc. This is highlighted in thesample pie-chart of FIG. 10, with details now provided.

Specifically, the DSP core may include a debugging memory that harvestsand stores a continuous stream of full rate data, for post-processingand diagnostic purposes. Typically, the ADC/equalizer output data(sampled synchronously at the Baud rate due to the timing recoveryloop), can be collected at the debug memory and passed to the diagnosticlogic residing in the MLS. The collected data can be used for at leastthe following.

Data collected at the debugging memory can be used to estimate linkslicer SNR. The PAM chip estimates slicer SNR internally. The estimatedSNR can be read through a register. The SNR is estimated as follows. Theequalizer output data (an example of which is shown in FIG. 6), issliced using the converged PAM levels and slicer thresholds (internallydriven by a DSP control-loop). The discrepancy between the equalizeroutput and the expected value (i.e., the PAM level) is called an errorsignal. The PAM SNR is computed as a simple ratio between the averagelevel power and the error signal mean square.

Data collected at the debugging memory can be used to estimate the totalchannel pulse response at the ADC/equalizer output. This can be done forexample, with a simple least-square channel estimation technique.Symbols for channel estimation can either be provided by the symboldecisions inside the DSP, or by a Pseudo-Random Binary Sequence (PRBS)generator where PBRS data is used for diagnostics. The estimated channelpulse response provides several pieces of information on the combinedlink ISI. An example of estimated pulse response, after equalization, isshown in FIG. 9. This affords indication on the residual ISI afterequalization, as can result from impedance mismatches, reflections . . .etc.

Data collected at the de-bugging memory can also provide non-linear ISI.Specifically, the estimated linear ISI component is first subtractedfrom the collected signal and serves as a non-linear ISI estimation.There are at least two ways to estimate non-linear ISI. A first approachis parametric, and the second is non-parametric. The parametric approachinvolves matching the residual signal to a specific form of non-linearISI given by nonlinearity model (e.g., third order nonlinearcompression, Volterra series . . . ). The nonparametric approachutilizes filling a look-up-table (LUT) with the average of the residualsignal for each data pattern.

Noise characterization can be obtained from data from the debuggingmemory. That is, once the linear and nonlinear components are extractedfrom the collected signal, the remaining signal is used for noisecharacterization. This includes estimating the noise variance andcorrelation per data pattern.

Finally, once the linear, nonlinear and noise components arecharacterized, it is possible to clearly point to the limitingfactor/factors in a given link. The pie chart of FIG. 10 is useful forthis purpose.

Receiver eye scan (e.g., under the CAUI 100 GbE standard) represents yetanother function potentially useful for diagnostic purposes. TheHost-side CAUI Rx(s) can support a 3D eye-scan feature. This is shown inFIG. 11.

It is to be appreciated that by using enhanced diagnostics according tovarious embodiments, the performance of a PAM module can be determinedand updated, thereby allowing high data transmission rate and low errorrate. Various embodiments can be implemented with existing systems.There are other benefits as well.

1A. An optical communication module comprising:

an optical transmitter element and an optical receiver elementconfigured to handle communication of optical data and electronic dataincluding digital signal processing (DSP); and a master logic controlcomprising a first register storing a first DSP parameter captured by adebugging memory, the master logic control configured to,receive from the optical transmitter element or the optical receiverelement, additional raw parametric information relating to the opticaldata or the electronic data,cause an engine to process first DSP parameter with the additional rawparametric information to create second diagnostic information,store the second diagnostic information in a new register, andsend the second diagnostic information to a host via a communicationsinterface.

2A. The optical communication module of clause 1A wherein processing thefirst DSP parameter with the additional raw parametric informationcomprises extracting a pulse link response highlighting a residuallinear Inter Symbol Interference (ISI) component.

3A. The optical communication module of clause 2A wherein the engine isfurther configured to break down a finite link Signal-to-Noise Ratio(SNR) into the residual linear ISI component and other components.

4A. The optical communication module of clause 3A wherein the othercomponents comprise noise, non-linearities, and low-frequency directcurrent (DC) baseline errors.

5A. The optical communication module of clause 4A wherein:

the DSP parameter comprises a Digital Timing Loop (DTL) parameter;

a Kf accumulator value is translated to extract lane frequency offset;and

the noise is extracted by accumulating DTL readout.

6A. The optical communication module of clause 4A wherein:

the DSP parameter comprises analog front-end gain; and

the DC baseline errors are determined from Continuous Time LinearEqualizer (CTLE) Equalization code to assess a peak-peak value andidentify DC gain and applied high-frequency analog boost.

7A. The optical communication module of clause 4A wherein:

the DSP parameter comprises analog front-end gain; and

the DC baseline errors are determined from DC offset and timing mismatcherrors.

8A. The optical communication module of clause 4A wherein the engine isfurther configured to process one or more of Feed Forward Equalization(FFE) tap coefficients, PAM4 levels, slicer thresholds, DecisionFeedback Equalization (DFE) tap coefficients, and reflection-cancellercoefficients.

9A. The optical communication module of clause 1A wherein:

the first DSP parameter comprises a Feed Forward Equalization (FFE)output;

the additional raw parametric information comprises slicer information;and

the second diagnostic information comprises a Bit Error Rate (BER) of aForward Error Correction (FEC) function.

10A. The optical communication module of clause 1A wherein the first DSPparameter comprises an Analog-to-Digital Conversion (ADC) output.

11A. The optical communication module of clause 1A wherein the seconddiagnostic information comprises an eye diagram.

12A. The optical communication module of clause 1A wherein the seconddiagnostic information comprises an eye histogram collected bypopulating post-equalization sample hits until a single bin saturates.

13A. The optical communication module of clause 1A wherein the seconddiagnostic information comprises a signal-to-noise ratio (SNR).

14A. The optical communication module of clause 13A wherein:

the first DSP parameter comprises a first error signal;

the additional raw parametric data comprises a second error signal fromthe DSP; and

the SNR is an estimated Mean Square Error (MSE) based upon the firsterror signal and the second error signal.

15A. The optical communication module of clause 1A wherein the seconddiagnostic information comprises a frame error.

16A. The optical communication module of clause 15A wherein the frameerror comprises a module frame error count or errored seconds.

17A. The optical communication module of clause 1A utilizing forwarderror correction (FEC), wherein the second diagnostic information isselected from a FEC bit error rate, a FEC mode, and a FECenable/disable.

18A. The optical communication module of clause 17A wherein the seconddiagnostic information comprises a bit error rate (BER).

19A. The optical communication module of clause 18A wherein furthercomprising another new register and the BER comprises a per-level BER.

20A. The optical communication module of clause 19A wherein above a FEClimit, the BER is estimated based on Signal-to-Noise Ratio (SNR).

1B. A method comprising:

collecting data in a debugging memory of an optical transceiver;

communicating the data to diagnostic logic of a controller; and

processing the data to produce a diagnostic output.

2B. The method of clause 1B wherein:

the data comprises equalizer output data; and

the diagnostic output comprises an estimated link slicer signal-to-noiseratio (SNR) produced by, slicing the equalizer output data usingconverged PAM levels and slicer thresholds internally driven by a DSPcontrol-loop,

labeling a discrepancy between the equalizer output and an expected PAMlevel as an error signal, and

calculating the estimated link slicer SNR as a ratio between an averagelevel power and an error signal mean square.

3B. The method of clause 1B wherein:

the data comprises equalizer output data; and

the diagnostic output comprises an estimated channel pulse responseproduced by a least-square channel estimation employing a symbolprovided by a symbol decision within a digital signal processor (DSP) ora Pseudo-Random Binary Sequence (PRBS) generator.

4B. The method of clause 3B further comprising determining a linearresidual linear Inter Symbol Interference (ISI) component from theestimated channel pulse response.

5B. The method of clause 4B further comprising determining a non-linearISI component by subtracting the residual linear ISI component from asignal.

6B. The method of clause 4B further comprising matching the residualcomponent to a form of non-linear ISI according to a nonlinearity model.

7B. The method of clause 4B further comprising filling a look-up-table(LUT) with an average of the residual component for each data pattern.

8B. The method of clause 4B further comprising:

extracting the linear ISI component and a non-linear ISI component froma signal to leave a remaining signal; and

performing noise characterization on the remaining signal by estimatinga noise variance and correlation per data pattern.

9B. The method of clause 1B wherein the diagnostic logic processes thedata to produce the diagnostic output.

10B. The method of clause 9B further comprising communicating thediagnostic output across an interface to a host for further diagnosticprocessing.

As has been described above, a number of additional parameters may besupported by embodiments with advanced modulation techniques. To allowmonitoring of these parameters a new page in the SFF-8436/SFF-8636memory space can be utilized. Here, a new Page 20h is assigned tomonitor these parameters.

The basic monitoring techniques will be the same as for other monitoredparameters (i.e., they will support current value, latched warning/alarmstatus, masks, and thresholds). For alarm and warning flags and masks,as in SFF-8436/SFF-8636 the highest bit number will be used forhigh-alarm, followed by low-alarm, high-warning, and low-warning indescending bit order.

For enhanced modulation techniques such as PAM-4, there may be adifferent number of optical channels than electrical channels.Accordingly, embodiments include a logical mapping feature to identifythose electrical channels operating on particular optical channels.

For modules supporting error counters a counter reset bit is provided atregister 191, bit 7 of the new Page 20h.

For a module implementing a Dense Wave Division Multiplexing (DWDM)optical interface, there may be a benefit in providing access toadditional diagnostic monitoring parameters specifically for a DWDMmodule. In DWDM, the wavelength and/or frequency of the laser representimportant parameters. Hence monitoring of laser frequency and/orwavelength allows the health of the laser to be known.

Frequency error may be determined by using deviation of the lasertemperature from target as a proxy. In addition, DWDM modules typicallyuse a TEC to control the laser temperature. The current flowing throughthe TEC is a strong indicator of the health of the module. A warning orerror indication in any of these parameters can be an early indicationof pending module failure.

Not all possible features will be supported by all modules or on allchannels. To address this situation, embodiments allow the module todetermine those parameters being monitored, and in association withwhich aspects of the module.

Some parameters may be module-level in scope, and some may bechannel-specific. Some may be ingress-path related and some egress-pathrelated.

In a particular embodiment, this information is conveyed by the moduleto the host in the parameter configuration registers 164-187 shown inFIG. 16 discussed later below. Up to eight independent parameters can bemanaged with up to eight different threshold values. Additionalparameters (up to twelve total in this embodiment) can be managed,provided that they share threshold values (e.g., a same parametermonitored on different channels).

Finally, to facilitate later-added functionality without requiringadditional specification changes, the parameter configuration registersmay support an enumerated value for the specific parameter to bemonitored. It is expected that the official list of which enumerationvalues correspond to what measurement and units, will be managed inSFF-8024 (similar to other enumeration values in the SFF-8436/SFF-8636specification).

A brief description of newly monitored parameters for PAM-4, is nowprovided. These monitored parameters have also been discussed previouslyabove.

One newly monitored parameter is SNR. This feature measures thesignal-to-noise ratio of the electrical data present on the channel.This parameter can be interpreted as 8.8 fixed decimal. For example, anMSB value of 13h and LSB value of 80h will be interpreted as an SNR of19.5.

Another newly monitored parameter is residual ISI/dispersion. Thisfeature measures the amount of correction being done by the module toaccount for residual inter-symbol interference. The usual cause for thisis optical dispersion, so this measurement is a proxy for residual(uncorrected) optical dispersion that is being corrected by the module.This parameter is unitless, and the threshold alarm and warning valueswill give an indication of the severity of the uncorrected dispersion.

Still another newly monitored parameter is the PAM Histogram parameter.This feature measures the rate of measured signal on the line that hasan analog level near the cutoff for a PAM bit transition (e.g., 0<->1,1<->2, 2<->3). This parameter is unitless and the threshold alarm andwarning values will give an indication of the magnitude of the rate ofanalog measurements near the cutoff points.

Note that for this parameter, there is no concept of a low alarm orwarning, so the corresponding thresholds will be 0. Note also that therecan be multiple transition points depending on the level of PAMencoding. For PAM-4 there will be 3 transition points. For PAM-8 therewould be 7. This parameter simply sums the data from all transitionpoints.

Yet another newly monitored parameter comprises error counters. Thisfeature monitors the error rate for either the full 100G link or on achannel-by-channel basis. Two separate error counters may be maintained:

-   -   Uncorrected BER since last counter reset;    -   Pre-FEC BER since last counter reset.

The parameters may be represented as accumulated BER where theaccumulation period is reset manually by the host. To do this, theregister on page 20h, address 192, bit 7, may be set to 1.

Note that the thresholds system is maintained for BER and uncorrectederrors. But, the low thresholds for BER and errors should be 0, and thehigh threshold for uncorrected errors should also be 0 unless othererror correcting schemes are present.

BER parameters may be interpreted as an unsigned 16-bit floating pointnumber with 5 bits for (base-10) exponent (offset by −24) and 11 bitsfor mantissa. Thus the format may be: m*10^(s+o)

where m ranges from 0 to 2047 (11 bits), s ranges from 0 to 31 (5 bits)and o is fixed at −24. Thus the smallest non-zero number is m=1 and s=0or 10⁻²⁴.

Certain parameters are newly monitored for DWDM. These are now discussedbriefly below, and have also been described in detail above.

The TEC current parameter may be monitored. This parameter monitors theamount of current flowing to the TEC of a cooled laser. In a particularembodiment, this parameter is a 16-bit signed 2s complement value with aLSB unit of 0.1 mA. Thus, the total range is from −3.2768 A to +3.2767A.

Laser frequency may also be monitored as a parameter. This parametermonitors the difference (in frequency units) between the target centerfrequency and the actual current center frequency. It is a similarmeasurement to the laser temperature (above), except expressed as afrequency difference instead of a temperature difference. Vendors maysupport one or the other measurement, or both. In an example laserfrequency may be a 16-bit signed 2s complement value with a LSB unit of10 MHz. Thus the total range is from −327.68 GHz to 327.67 GHz.

Laser temperature may also be monitored as a parameter. This parametermonitors the laser temperature difference between the target lasertemperature for a cooled laser, and the actual current temperature. Itis a similar measurement to the frequency error, except expressed as atemperature difference instead of a frequency difference. Again, vendorsmay support one or the other measurement, or both. In an example lasertemperature may be a 16-bit signed 2s complement value with a LSB unitof 0.001° C. Thus the total range is from −32.768° C. to 32.767° C.

The various additional parameters now being monitored according toembodiments, may be stored in a memory space. In this particularexample, a new page (Page 20h) in the SFF-8436/SFF-8636 memory space canbe utilized for this purpose.

FIG. 12 is a table showing a register overview for the new page 20h.FIG. 12 shows that registers 128-133 of Page 20h are used to storelatched alarm/warning flags for the monitored parameters.

FIG. 13 is a table showing registers for alarm/warning latches. Inparticular, these six bytes cover the latched alarm and warning flagsfor the monitored parameters specified by the parameter configurationregisters. Each parameter has four bits, with the most-significant bit(MSB) representing the alarm high error, followed by alarm low, warninghigh, and warning low as with other alarm and warning flags. Note thatthe threshold against which the real-time value is compared to generatethese alarms and warnings is specified in the Parameter ConfigurationRegisters.

Returning to FIG. 12, in that table the registers 134-157 of the newPage 20h are used to store real-time values of monitored parameters.FIG. 14 is a table showing real-time value registers.

These twenty-four bytes contain the real-time value of the monitoredparameters. They are to be interpreted as specified in the ParameterConfiguration Registers. In addition, the module will compare thesevalues to the corresponding thresholds indicated in the ParameterConfiguration Registers to generate the appropriate alarms and/orwarnings in the registers 128-133 described above. As withSFF-8436/SFF-8636, these parameters are stored with the MSB in the lowernumbered address and the least-significant bit (LSB) stored in thehigher numbered address.

Returning to FIG. 12, in that table the registers 158-163 of the newPage 20h are used to store interrupt mask values for monitoredParameters. FIG. 15 is a table showing interrupt mask registers.

These six bytes cover the interrupt masks for the latched alarm andwarning flags. Each parameter has four bits, with the most-significantbit representing the alarm high error, followed by low alarm, highwarning, and low warning as with other alarm and warning parameters.When a particular bit is a zero, then the corresponding flag willgenerate an interrupt if asserted. If the bit is one, then the interruptwill not be generated. As with the alarm and warning flag, for eachparameter the highest bit number represents alarm high followed by alarmlow, warning high and warning low masks.

Returning to FIG. 12, in that table the registers 164-187 of the newPage 20h are used to store Parameter Configuration Registers. FIG. 16 isa table showing parameter configuration registers.

This section of the register map determines how the real-time valueregisters, alarms and warnings, masks and thresholds are to beinterpreted by the host. For each of the twelve possible monitoredparameters, the monitoring point, parameter type, and threshold locationare provided by the module. The parameter configuration is a 2-bytefield which is now described.

In particular, the two bytes of the parameter configuration are storedMSB-first (MSB having the lower numbered address), defined according tothe table of FIG. 17 showing parameter configuration details.

The parameter type value of FIG. 17, is taken from the table of FIG. 18showing parameter type enumeration. In certain embodiments this table isstored in SFF-8024 along with other enumeration tables.

For enhanced modulation techniques such as PAM-4, there may be adifferent number of optical channels than electrical channels. Forexample, the simplified view of FIG. 2 above, shows the optical modulehaving two electrical channels and one optical channel.

Accordingly, embodiments store mappings that allow the host to retrievethe electrical to optical channel mapping. For a PAM-4 encoding, theelectrical channel can be either mapped to the MSB or the LSB of theoptical channel. FIG. 19 is a simplified diagram illustrating mapping bythe optical module between a plurality of optical lanes 1900 and aplurality of electrical lanes 1904.

Each electrical channel has a 4-bit register in register 188 or 189 todefine this mapping. FIG. 20A is a table showing lane mapping registersof the Lane mapping parameter 1200 of FIG. 12. The mapping may bedefined as in the table of FIG. 20B showing lane mapping enumeration.This table may be stored in SFF-8024 as with other enumeration tables.

In particular embodiments, this mapping parameter is read-only.According to alternative embodiments, however, this parameter may bemodified to allow active configuration of the mapping between opticaland electrical channels.

Returning to FIG. 12, in that table the registers 190-191 of the newPage 20h are used to store other configuration registers. FIG. 21 is atable showing other configuration registers according to an embodiment.

In particular, this section contains a single bit that allows the hostto reset the module error counters so that a recent BER can bepresented. Its implementation is optional. If the module automaticallyrestarts its error counters, then it is not necessary for the host to doso.

Returning again to FIG. 12, in that table the registers 192-255 of thenew Page 20h are used to store alarm/warning thresholds. FIG. 22 is atable showing threshold registers.

In particular, this section contains the threshold registers againstwhich the various parameters will be compared to determine if an alarmor warning flag should be generated. Each threshold set has four 2-Byteregisters ordered MSB-first (lower numbered address contains MSB), andthe registers are in the same order as other threshold registers inSFF-8436/SFF-8636: alarm high threshold, alarm low threshold, warninghigh threshold, warning low threshold.

While the above specification is a full description of the specificembodiments, various modifications, alternative constructions andequivalents may be used. Therefore, the above description andillustrations should not be taken as limiting the scope of the presentinvention which is defined by the appended claims.

What is claimed is:
 1. An apparatus comprising: a receiver configured tohandle communication of optical data received over a plurality ofoptical channels and a parameter of the optical data communicated over aplurality of electrical channels; a debugging memory configured tocollect and store the parameter from the receiver; and a master logiccontrol comprising a first register storing the parameter of the opticaldata captured by the debugging memory, the master logic controlconfigured to, receive from the receiver parametric information relatingto the optical data, store the parametric information; process theparameter with the parametric information to create diagnosticinformation, and store the diagnostic information in a second register.2. The apparatus of claim 1 further comprising a number of the pluralityof optical channels that differs from a number of the plurality ofelectrical channels, the parametric information further specifying achannel mapping value.
 3. The apparatus of claim 2 wherein the channelmapping value is actively configurable.
 4. The apparatus of claim 2wherein the optical data is encoded by PAM4.
 5. The apparatus of claim 1wherein the parameter specifies a PAM4 histogram parameter.
 6. Theapparatus of claim 1 wherein the parametric information further includesa value specifying parameter monitoring at a host side or at a lineside.
 7. The apparatus of claim 1 wherein the parametric informationfurther includes a value specifying parameter monitoring at an ingressside or at an egress side.
 8. The apparatus of claim 1 wherein theparametric information further includes a value specifyingchannel-specific parameter monitoring.
 9. The apparatus of claim 1wherein the parametric information specifies an optical source propertycomprising a frequency, a temperature, or a current.
 10. The apparatusclaim 1 wherein the parametric information indicates a bit error rate.11. The apparatus of claim 1 wherein the parametric informationindicates a signal-to-noise ratio.
 12. A method comprising: a debuggingmemory receiving from an optical receiver, parametric informationrelating to optical data incoming to the receiver; streaming theparametric information from the debugging memory to a first register ofa master logic system; processing the optical data with the parametricinformation by the master logic system to create diagnostic information;and storing the diagnostic information in a second register of themaster logic system.
 13. The method of claim 12 wherein the parametricinformation further includes a value specifying a threshold.
 14. Themethod of claim 12 wherein the optical data is encoded by PAM4.
 15. Themethod of claim 12 wherein the parametric information further includes avalue specifying monitoring at a host side or at a line side.
 16. Themethod of claim 12 wherein the parametric information further includes avalue specifying monitoring at an ingress side or at an egress side. 17.The method of claim 12 wherein the parametric information furtherincludes a value specifying channel-specific parameter monitoring. 18.The method of claim 12 wherein the parametric information specifies anoptical source property comprising a frequency, a temperature, or acurrent.
 19. The method of claim 12 wherein the parametric informationindicates a bit error rate.
 20. The method of claim 12 wherein theparametric information indicates an amount of correction to account forresidual inter-symbol interference.